chore(onyx-207): migrate off Webpacker to Shakapacker

[nickskalkin]

Ticket

Webpacker was deprecated by Rails team and no longer receives support. There are a few alternatives to it, one of them is Shakapacker. We've agreed to switch to it, mostly because migration from Webpacker to Shakapacker is easier than to other gems.

I was following this migration guide.

Checklist

• [x] Local development with hot reloading works
• [x] Tests, linters pass
• [x] Assets compilation works